The Insulated-Gate Bipolar Transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch and in newer devices is noted for combining high efficiency and fast switching. It switches electric power in many modern appliances such as: Variable-Frequency Drives (VFDs), electric cars, trains, variable speed refrigerators, lamp ballasts, air-conditioners, and even stereo systems with switching amplifiers.
IGBTs are often used for high voltage (e.g., greater than 600V) and high-current power converter applications. In these types of applications, a short-circuit of the load wire to a power source will result in a large current flowing through the IGBT, which is likely to damage the IGBT. Because of the potential for damage to IGBTs, gate drive circuits must detect IGBT short-circuit conditions and turn off the IGBT safely to prevent damage to the IGBT.
Depending upon the types of IGBTs employed in a system, there are two common approaches to detect short-circuit events for IGBTs: (1) detection of the IGBT's collector-to-emitter desaturation voltage and (2) detection of a fraction of the IGBT's emitter current. Detecting the collector-to-emitter desaturation voltage is useful when access to the IGBT collector node is available. On the other hand, emitter current sensing is useful for current sense IGBTs where there is an additional sense node split out from the emitter node of the IGBT.
A common IGBT drive circuit 100 without short-circuit protection is shown in FIG. 1 where a load 112 is driven by current from IGBTs 108, which are in turn driven by gate drivers 104. The depicted circuit 100 is often referred to as a half-bridge circuit and is among the most important circuit configurations for power drives. The circuit 100 is shown to include two IGBTs 108 connected to one another at the circuit's 100 midpoint and the load 112 is connected to this midpoint. The midpoint corresponds to a circuit node where an emitter E of one IGBT 108 is connected to a collector C of another IGBT 108.
Problematically, as shown in FIG. 1, if the circuit 100 experiences a short (e.g., between Ground/common voltage and the circuit's 100 midpoint) then excessive current will flow through the top IGBT 108, most likely resulting in damage to the IGBT 108.
FIG. 2A depicts an illustrative circuit 200 that includes a short-circuit detection circuit 208 that enables the detection of a short-circuit event that could potentially damage the first IGBT Q1 in the circuit 200. The circuit 200 facilitates the detection of collector-to-emitter desaturation voltage of the first IGBT Q1. It should be appreciated that the first IGBT Q1 may correspond to the same or similar component as the IGBT 108 depicted in FIG. 1. Similarly, the second IGBT Q2 depicted in circuit 200 may correspond to the same or similar component as the IGBT 108 depicted in FIG. 1.
As in FIG. 1, the circuit 200 is configured in a half-bridge configuration where a first driver 204a is driving the first IGBT Q1 and a second driver 204c is driving the second IGBT Q2. The short-circuit detection circuit 208 is shown to include a second driver 204b that senses the collector-to-emitter voltage Vce for the first IGBT Q1. An output of the second driver 204b provides information back to the first driver 204a such that if the short-circuit detection circuit 208 detects a short-circuit event, the first driver 204a is turned off. Most often, the short-circuit event that is detected by the short-circuit detection circuit 208 corresponds to a short between the midpoint 216 of the half-bridge circuit and ground. When such a condition is detected, the second driver 204b provides a signal to the first driver 204a that causes the first driver 204a to turn off. The second driver 204a is turned off in an effort to protect the first IGBT Q1 from overheating and/or damage due to increased current flowing from the collector C to the emitter E (known as collector-to-emitter current Ice).
During short-circuit protection test in circuit 200 for a short-circuit event, the third driver 204c is kept inactive, thereby keeping the bottom side of the second IGBT Q2 off. It should be appreciated that while circuit 200 is shown as including two IGBTs Q1, Q2, a circuit with a greater or lesser number of IGBTs may benefit from short-circuit detection and protection techniques.
During turn on of the IGBT Q1, the gate-to-emitter voltage rises from zero or negative supply and enters the Miller Plateau level. The Miller Plateau is caused by Miller Current flowing through the IGBT, which can be represented as dVCE/dt*CGC. During the Miller Plateau region, the collector-to-emitter voltage of the IGBT is larger than the desaturation voltage threshold, therefore the IGBT short-circuit detection circuit 208 has to be blanked from this region in order to avoid false triggers (i.e., false positive detection of a short-circuit event resulting in an unnecessary shut-down of the IGBT).
Known blanking circuits used for the circuit 200 utilize a capacitor, Cblank, shown in FIG. 2B to create a fixed blanking time. A problem with the known blanking solutions is that the fixed blanking time have to be relatively large based on the worst case scenario of the Miller Plateau to avoid the false positive detection of a short-circuit event. In addition practical tolerance of circuit parameters, e.g. charging current Ichg, capacitor tolerance due to Cblank and voltage threshold tolerance due to Vdesat_th have to be accommodated by designed minimum blanking time, which results in over-design of total blanking time. Ultimately, most durations of the Miller Plateau are not as long as the worst case scenario. Thus, the fixed amount of blanking time causes slow responses to short-circuit events, which could ultimately result in high short circuit current and shorten life expectation of IGBT.
FIG. 3A depicts another illustrative circuit 300 that includes a short-circuit detection circuit 208 that enables the detection of a short-circuit event that could potentially damage the first IGBT Q1 in the circuit 300. The circuit 300 differs from circuit 200 in that circuit 300 is configured to monitor sense current Is flowing through the emitter of the first IGBT Q1. Specifically, circuit 300 utilizes a short-circuit detection circuit 208 to measure current Is flowing from the emitter 304 of the first IGBT Q1. The emitter 304 of the first IGBT Q1 is different from the emitter of the first emitter in circuit 200 in that the emitter 304 of the first IGBT Q1 is a split emitter that emits an emitter current Ie and a sense current Is. The sense current Is is a fractional value of the emitter current Ie. The short circuit detection circuit 208 comprises a resistor that creates a voltage that can be measured by the second driver 204b and compared with a threshold voltage value. If the measured voltage exceeds the threshold voltage value, then the second driver 204b determines that a short-circuit event is occurring and turns off the first driver 204a, thereby turning off the first IGBT Q1.
A problem in emitter sense detection is that due to high collector to emitter voltage during Miller plateau, there are spikes or blips of finite duration that appear on the sense voltage (Vs) These over voltage on Vs may cause the false triggering (i.e., false positive detection of a short-circuit event). Blanking solutions have also been deployed for the emitter sense detection circuit 300. Much like blanking solutions for circuit 200, the currently-available blanking solutions for circuit 300 utilize an RC filter, e.g. Rf and Cf as shown in FIG. 3B, to create a fixed filter time constant. Again, the resistance and capacitance of the filter are relatively large and designed for the worst case scenario. The over design of the filter circuit to accommodate over voltage on Vs during Miller plateau can cause slow response time, which limit the utility of short-circuit detection circuits.